A fuel cell system, which is a main power source for a fuel cell vehicle, is an apparatus that generates electric energy through an oxidation-reduction reaction of hydrogen and oxygen. High purity hydrogen is supplied from a hydrogen storage tank to an anode of a fuel cell stack (hereinafter, referred to as a “stack”), and air in the atmosphere supplied by an air compressor or other air supply devices is introduced into a cathode of the stack.
An oxidation reaction of hydrogen is progressed in the anode to generate hydrogen ions (protons) and electrons, and the generated hydrogen ions and the generated electrons are moved to the cathode through a polymer electrolyte membrane and a separator plate, respectively. Further, a reduction reaction of the hydrogen ions and the electrons moved from the anode and oxygen in the air supplied by an air supply device is progressed in the cathode to generate water and, at the same time, generate electric energy by flow of the electrons.
Meanwhile, to purge hydrogen and discharge condensate, a fuel cell system according to the related art includes a purge valve configured to selectively discharge hydrogen and other gases circulating in the anode to the outside, a water trap in which condensate discharged from an anode is stored, and a condensate valve configured to selectively discharge the condensate stored in the water trap to the outside. The purge valve and the condensate valve are connected to an air discharge line through which humid air discharged from the stack may be discharged to the outside or a humidifier configured to humidify air to be supplied to the stack using the humid air. Accordingly, the hydrogen and other gases discharged from the purge valve and the condensate discharged from the condensate valve may be discharged to the outside along the air discharge line.
However, when the hydrogen and other gases discharged from the purge valve are discharged to the outside along the air discharge line as provided by the related art, a hydrogen concentration of exhaust gas increases, which thus increases a risk of violating a regulation on the hydrogen concentration in the exhaust gas. Further, the hydrogen and other gases circulating in the anode as well as the condensate may be discharged through the condensate valve. Accordingly, when the hydrogen and other gases discharged from the condensate valve are discharged from to the outside along the air discharge line, there is also a risk of violating the above-described regulation on the hydrogen concentration. To prevent such violation of the regulation on the hydrogen concentration, a technique in which hydrogen is purged and condensate is discharged in a state in which an air pressure control valve installed in the air discharge line is closed has been used.
In addition, the air discharge line and the humidifier are connected, through the cathode, to an air supply line configured to supply air to the cathode, and such an air supply line is connected, through a bent tube, to a stack enclosure in which the stack is received Particularly, the bent tube transfers a negative pressure provided by an air compressor installed in the air supply line to the stack enclosure. Then, leakage gas leaked from the stack and received in the stack enclosure is suctioned by the negative pressure provided by the air compressor, and is then discharged to the outside sequentially via the bent tube, the air supply line, the cathode, the air discharge line, and the like.
When power generation is stopped while the fuel cell system is operated, the air compressor is stopped. Accordingly, when the hydrogen is purged and the condensate is discharged in a state in which the air compressor is stopped, there is a concern in that the hydrogen flows backwards to the stack enclosure along the humidifier, the air discharge line, the cathode, the air supply line, the bent tube, and the like. When the hydrogen flows backwards to the stack enclosure, a hydrogen leak from the stack may be erroneously detected, a hydrogen detection sensor may be damaged, or a safety problem due to hydrogen accumulation may occur. Accordingly, in the related art, a technique in which a check valve or another backflow prevention member is installed in the air discharge line or the like to prevent the hydrogen from flowing backwards to the stack enclosure has been considered. However, in this way, when the backflow prevention member is installed, installation costs of the fuel cell system increase substantially.